Problems with AC - can't get temp right

I have central air in my house and the unit and everything is about 20 y/o. I realize it should be updated but I have no control over that.
The problem I have is that if I set the thermostat to to 70 degrees fahrenheit, then the downstairs stays about 70. But the upstairs will be anywhere from 76-80, depending on the room and how hot it is outside. So the problem is that people complain that it's too cold downstairs and too hot upstairs. Ideally, I would like it to be about 76-78 downstairs and about 74 upstairs.
Why does this happen and is it normal? I know that a lot of air blows out in the vents downstairs and not a lot blows out upstairs. I shut off all the vents that aren't needed and close the doors to rooms that aren't used. I have the fan set to "auto". I don't know if leaving it on auto is better than setting it to "on". I don't like to mess with it because it seems to cause problems with it.
Any opinions or suggestions (other than buy a new unit)?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

You need to balance the system better, but you are on the right track with what you have been doing. . First, be sure all filters are clean, no ducts shut off. Be sure the belt on the blower is not loose and slipping. Does it have to be lubricated? Next, close or partially close the downstairs vents as you have been doing to force more air upstairs. Check returns to see that they are not blocked also.
During the day, the upstairs is subjected to much more heat from the sun on the roof. You may bet better results if you vent the attic better and perhaps add some insulation.
Turning the fan to the "on" position will make it run all the time. That may help distribute the air more evenly, but at the cost of running the blower all the time. Closing the doors to rooms not used may help or hinder, depending on how well the air is flowing around the house.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mike S. wrote:

You are on the right track. Follow the advice Ed provided. I will only add this. The real answer you have ruled out in that you may need changes to the distribution (supply and return ducts) system and maybe increased air handling capacity. That requires some measurements and math. There are standard formulas for that. There is only so much you can or should do with closing off vents. Close too many can you can cause damage to the system.
You may be in a no win situation as the lack of the ability to make serious changes to the system may rule out the only real answers.
--
Joseph Meehan

Dia \'s Muire duit
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joseph Meehan wrote:

Both poster's gave excellent advice. It is probably seldom that a manual J is done to get an accurate BTUH heatgain for Each Room, and then a manual D performed to achieve the proper air CFM Delivery and Return to Each Room, whether the room is on the first or second floor areas. It can be done right but will cost more, even during the initial installation, due to added duct & labor.
If you want the entire top floor cooled, if there are "Adequate Air Returns to the airhandler," it will help to seal off the stairway at the end of the top floor hallway. That could be done with a temporary summer only setup, or a permanent doorway installed. Are there dampers in the ducts near the airhandler? If the refrigerant control is a TXV, it will help to keep the E-Coil from slugging the compressor due to a lowered airflow caused by dampering off some rooms or partially covering the first floor Air Returns. You have to use caution when doing any of these things and you must always have the input advice of a competent HVAC Tech that knows all the aspects of that system! I am NOT responsible for what you decide to do. http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html - udarrell
--
WISDOM PRINCIPLE DIRECTED EMPOWERMENT COMMUNICATIONS -
THE REAL POLITICAL ISSUES and WISDOM BASED PEOPLE EMPOWERMENT
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 07 Jun 2007 22:50:55 -0700, Mike S. wrote:

Need more airflow upstairs. If you can close the downstairs vents half way. Play around with it but leave the stat on auto. 20 years is pushing it for a cooling system so there probably is an issue with cooling capacity.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Lots of good advice in other posts. You should be aware that "buy a new unit" will not solve your problem of unbalance between the heat loads and the air delivery between the two floors unless the airflow is changed. A two zone system or two separate systems would probably be much more satisfactory.
You may have the best low cost results with insulation, awnings or other shading, storm windows, caulking, or other means of reducing the heat gain in the upstairs.
Don Young
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
if it used to cool the upstairs ok, then you may need to have the cooling coil and outside coil cleaned to get better cooling/air flow. ive found shutting off vents often cuts back on that full air flow you need for cooling.also, if youve got a power vent in the roof thats not working it can cause higher temps upstairs. lucas
http://www.minibite.com/america/malone.htm
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mike S. wrote:

It sounds like the t'stat is doing exactly what you have asked it to do. Problem is with moving hot air out of upstairs. Any way to set up a small exhaust fan operating on a t'stat for the upstairs? 70 is darn cold, IMO. Depending on how the house is arranged, and the room for more air flow, perhaps a ceiling fan upstairs (in stairwell?) to move air up or down as needed to get temp more balanced
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.